Motor control circuit

ABSTRACT

Alternating current rectified by a bridge rectifier is supplied through an SCR device to a motor armature; the bridge rectifier, SCR device, and armature being connected in a series loop circuit. A firing control circuit associated with the SCR device controls the angle at which the device fires during each half cycle of alternating current received from the bridge rectifier. A storage capacitor and a freewheeling diode are coupled in parallel with the armature to assist in filtering. A resistor connected in series with the armature produces a feedback voltage signal which is proportional to armature current, and a resistive circuit branch arranged in parallel with the armature provides a feedback voltage signal which is proportional to armature voltage. A signal responsive circuit combines the two voltage feedback signals into a composite control signal for controlling the firing control circuit previously referred to. The firing control circuit is connected between anode and cathode of the SCR device for receiving its operating energy when the SCR device is nonconductive.

United States Patent Inventor Raymond 1' Mason Primary Examiner-Otis L.Rader y Assistant Examiner-H. Huberfeld l PP 831,954 Aitorney-Beehlerand Arant [22] Filed Dec. 4, 1969 [45] Patented Aug. 24, 1971 [73] A iMi n-1k El t i Company ABSTRACT: Alternating current rectified by abridge rectifi- L A el C lif, er is supplied through an SCR device to amotor armature; the bridge rectifier, SCR device, and armature beingconnected in a series loop circuit. A firing control circuit associatedwith the SCR device controls the angle at which the device fires duringeach half cycle of alternating current received from the [54] MOTORCONTROL CIRCUIT bridg r ectifiel'- 16 Claims 2 Drawing Figs. g A storagecapacitor and a freewheeling diode are coupled in parallel with thearmature to assist in filtering. A resistor con- [52] US. Cl 318/308,nected in series with the armature produces a feedback w 318/311 318/345318/504 318/507 age signal which is proportional to armature current,and a re- [51] lltLCl. H02p 5/16 sistive i i branch arranged in parallelwith the armature [50] Field of Search 318/317, id a f db k voltagesignal which is proportional to ar- 504i 505i 303 mature voltage. Asignal responsive circuit combines the two [56] References Cited voltagefeedback signals into acomposite control signal for controlling thefiring control circuit previously referred to. UNITED STATES PATENTS Thefiring control circuit is connected between anode and 3,373,331 3/1968Dow 318/317 X cathode of the SCR device for receiving its operatingenergy 3,475,672 10/ 1969 Oltendorf 318/317 X when the SCR device isnonconductive.

IIIII Sue/v7 5 EE F1540 l lAAlA l l l l l l l MOTOR CONTROL CIRCUIT Thepurpose of the above abstract is to provide a nonlegal technicalstatement of the disclosure of the contents of the instant patentapplication and thus serve as a searching-scanning tool for scientists,engineers and researchers. Accordingly, this abstract is not intended tobe used in understanding or otherwise comprehending the principles ofthe present invention hereinafter described in detail, nor is itintended to be used in interpreting or in any way limiting the scope orfair meaning of the claims appended hereto.

BACKGROUND OF THE INVENTION The present invention is an improvement overmy Pat. No. 3,327,195 entitled Regulated Power Supply Circuit UtilizingControlled Rectifier issued June 20, 1967; and also over my Pat. No.3,447,055 entitled DC Motor Speed Control Circuit issued May 27, 1969.

In controlling the operation of a small DC motor it is desirable toincorporate into a single circuit package the necessary circuitry forrectifying the alternating supply current, for smoothing the rectifiedcurrent, for selecting the voltage applied to the motor and hence theoperating speed of the motor, and for regulating the motor voltage asthe supply line voltage and mechanical load on the motor vary. Thecomplete motor controller circuit can be conveniently incorporated in asingle package, and in usage it is simply plugged into a standard ACoutlet, connected to the DC motor, a speed selection knob is set to adesired speed setting, and a switch is then turned on to energize themotor. Since a particular controller may be manufactured to accommodateseveral different sizes or styles of motors, there are other settings oradjustments which may be required depending upon the particular motorwith which the controller is being use Another desirable circuit featuremay be used to protect the motor against excessive torque loads. Forexample, it may be desired to limit the armature current to 200 percentof the rated current. A torque limiting circuit of this kind is shown inmy Pat. No. 3,447,055 in FIG. 2 thereof, where transistor Q performs thecurrent limiting function.

In order to maintain the speed of the motor constant as the mechanicalload on the motor increases, it is necessary to actually increase thevoltage applied to the armature. Imperfect filtering of the rectifiedcurrent is deliberately used because of the high cost of induction coilsand capacitors. The control loop that is required for maintaining motorspeed constant therefore involves an aspect of positive or regenerativefeedback, with its attendant possibilities of hunting or oscillation, asis clearly explained in Pat. No. 3,134,065 issued to William J. Minarik,as well as in my prior Pat. No. 3,447,055.

THE PRESENT INVENTION Oneobject of the invention is to provide a motorcontrol circuit of the foregoing type, which is so arranged that theneed for some of the more expensive components is DRAWINGS FIG. 1 is aschematic circuit diagram illustrating one embodiment of the invention;and

FIG. 2 is a schematic circuit diagram illustrating a second embodimentof the invention.

Reference is now made to FIG. 1 of the drawings illustrating a firstembodiment of the invention. The circuit of FIG. 1 accomplishes the samegeneral purposes as the circuits shown in my Pat. Nos. 3,327,195 and3,447,055. That is, a single circuit incorporated into a single packageis utilized as a power supply circuit for converting alternating currentinto direct current, for selecting the speed at which the associatedmotor will operate, for regulating the power supply voltage againstsupply line voltage fluctuations and alsov for maintaining the motorspeed constant as the load on the motor varies.

The AC input power at the conventional 1 10 volts is applied toterminals 10, 11, passing directly from terminal 11 to one side of abridge rectifier l2 and from the terminal 10 through a choke coil L tothe other side of rectifier 12. The bridge rectifier 12 includessemiconductor diodes D D D and D and a positive DC supply line 13extends from the positive output tenninal of the bridge rectifier whilea negative DC supply line 14 extends from the negative terminal. A DCmotor armature A and an SCR rectifier device 15 are coupled into aseries loop circuit with each other and with the bridge rectifier 12.Bridge rectifier 12 operates continuously while the SCR device operatesintermittently in order to control the average voltage applied to themotor armature.

More specifically, the anode of device 15 is connected to positive lead13 and a potentiometer P is connected to its cathode. The other end of Pis connected to one side of the armature A, and a series field windingis coupled between negative line 14 and the other side of armature A. Aswitch S is connected in parallel with the series field winding so thatthis particular field winding may be switched into or out of the circuitas desired. A shunt field winding and a switch S1 are connected inseries with each other and between the lines 13, 14 so that the shuntfield also may be switched in or out as desired. Thus, the same controlcircuit may be utilized. for controlling either a series motor, a shuntmotor, or a seriesshunt motor. A freewheeling diode D, has its anodeconnected to line 14 while its cathode is connected to the cathode ofdevice 15. A storage capacitor C is also connected between lead 14 andthe cathode of device 15. A resistor R has one end connected to thecathode of device 15, and a resistor R is connected between the otherend of R and the line 14; that is, the series combination of R and R areconnected in parallel with the series combination of P the motorarmature, and the series field. The movable tap on potentiometer P isconnected to a lead 16 while the juncture of R and R is connected to alead 17.

The circuit of the present invention utilizes two separate voltagefeedback signals, in a manner generally similar to that disclosed in myPat. No. 3,447,055. Thus, the output lead 17 produces a first voltagefeedback signal which is substantially proportional to the voltageapplied across the motor armature. The output lead 16 produces a secondvoltage feedback signal which is substantially proportional to thecurrent drawn by the motor armature. Both of these voltage signals aremeasured relative to the cathode of device 15, to which a voltagereference lead 27 is connected.

In place of potentiometer P a fixed resistor may be used; however, themovable tap of the potentiometer permits adjustment of the amount offeedback as may be appropriate for a particular motor with which thespeed control circuit is being used.

The circuit of FIG. 1 also includes a signal responsive circuit 20, andthe leads l6, 17 which serve as output leads for deriving feedbacksignals from the motor armature also act as input leads for the signalresponsive circuit 20. The specific circuit of signal responsive circuit20 will be described subsequently.

The circuit of FIG. 1 further includes a control means 25 coupled inparallel with the SCR device 15, for selectively initiating current flowtherethrough. More specifically, a control lead 26 is connected to thegate terminal of SCR device 15,

and the voltage on lead 26 measured relative to the voltage on lead 27serves as the output signal from the control means 25. Operating energyfor the control means 25 is provided by the DC voltage existing betweenpositive supply lead 13 and reference voltage lead 27.

The function of signal responsive circuit 20 is to receive both of thevoltage feedback signals and produce a composite control signal, whichis delayed with respect time so as to avoid undesired hunting oroscillations, and therefore constitutes a function of a number of cyclesof both feedback signals. The composite control signal produced incircuit 20 is supplied on lead 22 to the control means 25. The functionof circuit 25 is to respond to that composite control signal and actuatethe gate terminal of the SCR device 15 for selectively initiatingcurrent flow to the motor armature.

Control circuit 20 includes, more specifically, a positive supply line21 to which a resistor R11 is connected. A potentiometer P is connectedto the other end of R11, the movable tap of P being connected to thejunction between R11 and P A potentiometer P, is connected between theremote end of P and lead 17. A resistor R, is also connected to lead 21,the other end of R10 being connected to the collector of an N-P-Ntransistor Q A resistor R is connected between the base of Q and themovable tap of P,. A resistor R is connected between lead 16 and theemitter of Q A capacitor C, has one terminal connected to lead 21, and aresistor R is connected between the other terminal of C and thecollector of 0,. Lead 22 is connected to the collector of Q representingthe output of circuit 20.

In the operation of circuit 20, the setting of P is used as a factorycalibration to provide *the desired voltage adjustment range for themotor control circuit. P is used as the speed selector for the motor.The first voltage feedback signal is supplied to the base of Q5 and thesecond voltage feedback signal is supplied to the emitter of hence O inconjunction with its biasing or load circuit operates to produce thecomposite control signal previously referred to. Capacitor C providesthe necessary storage or integrating function so that the compositesignal appearing on lead 22 at any particular instant of time is afunction of a plurality of preceding cycles of both of the feedbacksignals provided on leads l6 and 17.

In the control circuit 25 a resistor R, is connected to positive lead13, a Zener diode Z being'connected between the other end of R, andreference voltage lead 27. Lead 21 is connected to the junction betweenR, and diode Z, hence the Zener diode provides a regulated voltage bothfor the signal responsive circuit 20 and to operate Q of control circuit25.

A resistor R is connected between lead 21 and the emitter of a P-N-Ptransistor Q A capacitor C is connected between the collector of Q andreference lead 27. A resistor R is connected between lead 22 and thebase of 0,, A resistor R is connected between positive lead 13 and thepositive base of a unijunction transistor 0,. A resistor R is connectedbetween reference lead 27 and the negative base of Q,. The control leadof unijunction transistor Q, is connected to the collector of 0,.

Thus the composite control signal produced by signal responsive circuit20 on lead 22 is supplied to transistor Q and controls the charging ofcapacitor C When the charge on C reaches an appropriate level,conduction is initiated in the unijunction transistor 0,, creating asurge ofcurrent through R and thus applying a positive pulse on line 26to the gate terminal of SCR device 15. Current conduction through deviceis therefore abruptly initiated.

Each time the SCR device fires the voltage between its anode and cathodedrops to a fraction of a volt, and the supply voltage between lines 13and 27 which is available for control circuit 25 drops accordingly. Thecontrol circuit 25, is for practical purposes, shorted out at this time,and remains so until the end of the particular half-cycle. At the sametime the operating voltage applied to signal responsive circuitincluding the transistor Q, is diminished. This greatly reducedoperating voltage for the transistors 0,, Q and (2;, while the SCRdevice is conducting protects the transistors from voltage transients.to which they might otherwise be exposed, and therefore protects themagainst damage.

At the end of the half-cycle of supply voltage the conduc-, tion of theSCR device stops, because of the disappearance of its driving voltage.The gate lead of the SCR device is essentially at the same potential asthe cathode. When the next halfcycle of supply voltage is appliedthrough the bridge rectifier 12 the SCR device does not initiallyconduct. However, current flows through resistor R, and the Zener diode,thus providing a standard operating voltage (typically 20 volts) fortransistor 0,. Current also flows through resistor R11 for controllingthe bias of transistor of Q and through resistor R for making thattransistor viable and responsive to its applied input signals. Whenthecharge on C reaches the proper level,

under control of Q and Q the unijunction transistor Q, breaks down andthus initiates conduction of the SCR device. Then the cycle repeatsitself.

SECOND EMBODIMENT order to thereby limit the maximum torque applied tothe motor..Other'circuit changes have also been made which arecompatible with the inclusion of transistor Q,. A potentiometer P, hasbeen incorporated in the circuit in series between the armature A andpotentiometer P Themovable tap of I is connected through a lead 18 to aresistor R, the other end of R, being connected to the cathode of asemiconductor diode D The anode of D is connected to the base of 0,. Abias resistor R is connected between positive voltage lead 21 and thebase of Q The upper end of resistor R instead of being connected to thecathode of SCR device 15 is connected to the movable tap ofpotentiometer P Lead 16 instead of being connected to the movable tap ofP is connected to the juncture between P, and P The emitter-collectorcurrent path of Q, is inserted in the lead 16 in series with resistor Rthe collector being connected to R, while the emitter is connected tolead 16. In addition, a capacitor C is connected between positivevoltage lead 21 andthe base of transistor 0,. A low-resistance resistorR is connected in parallel with P and P, to carry the main part of thearmature current. Diode D is shunted across the armature and seriesfield.

In the operation of the circuit of FIG/2 the'voltage signal whichcontrols the torque-limiting transistor Q is developed acrosspotentiometer 1, between the leads 16 and 18.

Transistor Q, is normally saturated, but when armature cur-' generallyparallel branches of the circuit. One circuit branch is the potentialexisting between lead 21 and 18, which is divided by the connection oflead 16 between P and P Normally the main bias current flows from lead21 through resistor R and the base -emitter junction of Q, to lead 16.The other circuit branch is that existing from lead 21 through resistorR diode D and resistor R,,, to lead 18. A small current normally flowsthrough this branch of the.circuit, and resistors R 'and R, provide avoltage division ratio, with the resulting voltage at the base of Qbeing consistent with forward biasing of the base-emitter junction. Whenarmature current increases beyond the normal value the voltage across Pincreases and the voltage across P increases, both in proportion to thearmature current. There is an increased voltage between lead 16 and lead21, and there is also an increased voltage between lead 18. and lead 16,but the ratio' of these voltages has changed. The voltage divideroperation of R,, and R, causes the potential applied to the base of Q,to drop so that the baseemitter. junction is back-biased.

The circuit action is sharpened and accentuated by the nonlinear actionof diode D Under normal load conditions when normal armature current isflowing the diode permits only a very small current to flow through RBut with increased armature current and increased voltage appearingacross P, the driving voltage for diode D is increased to more than 0.6volt, with the result that a far greater current flows through the Rbranch of the circuit. This increased current flow produces a sharp dropin potential on the base of 0,, thus producing a sharply cutoffreverse-biasing action.

In the torque limiting circuit of FIG. 2 the movable tap of P, may beadjusted for the particular level of maximum armature circuit that isdesired. The maximum current may, of course, vary depending upon theparticular size and style of motor with which the control circuit isbeing used.

In the circuit of FIG. 2 typical components are:

D 1 amp. 600 v. silicon diode R 560 K ohms, l/2 w.

R 18 K ohms, 1/2 w. P, 50 ohms, l-l/2 w. pot. R 1 ohm, 10 w. (for 1/8I-I.P. motor) The torque control circuit of FIG. 2 has a much sharpercutoff than the type of torque control circuit which is shown in FIG. 2of my Pat. No. 3,447,055, and works a great deal better.

What I claim is:

l. A motor speed control circuit comprising: 4

an SCR device coupled in a series loop circuit with a motor armature forcontrolling the energy flow thereto; means for applying a rectified butpulsating voltage across said series loop circuit;

means for producing a first voltage feedback signal substantiallyproportional to the voltage applied to the motor armature;

means for producing a second voltage feedback signal substantiallyproportional to the current drawn by the motor armature; signalresponsive means responsive to both of said feedback signals forproducing a composite control signal as a function of a plurality ofcycles of both of said feedback signals; I

and control means coupled in parallel with said SCR device and energizedby the voltage existing betweenanode and cathode thereof, said controlmeans having an input coupled to said signal responsive means forreceiving said composite control signal and having an output coupled tothe gate terminal of said SCR device for selectively initiating currentflow through said SCR device, whereby each time that said SCR devicecommences to conduct the voltage between its anode and cathode drops toabout a fraction of a volt and the energizing voltage for said controlmeans drops accordingly.

2. A motor speed control circuit as claimed in claim 1 wherein saidsecond voltage feedback signal producing means includes a resistorcoupled in series with the motor armature, and which further includes acapacitor coupled in parallel with the series combination of saidarmature and resistor.

3. A motor speed control circuit as claimed in claim 2 which furtherincludes a diode coupled in parallel with said capacitor and poledoppositely to said SCR device.

4. A motor speed control circuit as claimed in claim 2 wherein saidsignal responsive means includes an electron discharge device havingfirst and second control electrodes coupled to said first and secondvoltage feedback signal producing means, respectively, and having anoutput electrode, and further including a storage capacitor coupled tosaid output electrode.

5. A motor speed control circuit as claimed in claim 2 wherein saidfirst voltage feedback signal producing means includes an additionalpair of resistors coupled in series with each other and in parallelacross said series combination of armature and resistor.

6. A motor speed control circuit as claimed in claim 4 wherein saidsignal responsive means includes an electron discharge device havingfirst and second control electrodes coupled to-said first and secondvoltage feedback signal producing means, respectively, and having anoutput electrode, and further including a storage capacitor coupled tosaid output electrode.

7. A motor speed control circuit as claimed in claim 1 wherein saidsignal responsive means includes an electron discharge device havingfirst and second control electrode coupled to said first and secondvoltage feedback signal producing means, respectively, and having anoutput electrode, and further including a storage capacitor coupled tosaid output electrode.

8. A motor speed control circuit as-claimed in claim 7 whereinsaid'control means includes a unijunction transistor, a firingcapacitor, a transistor regulating current flow to said firingcapacitor, and a Zener diode regulating the voltage supplied to theseries combination of said firing capacitor and transistor.

9. A motor speed control circuit as claimed in claim 1 wherein saidcontrol means includes a unijunction transistor, a firing capacitor, atransistor regulating current How to said firing capacitor, and a Zenerdiode regulating the voltage supplied to the series combination of saidfiring capacitor and transistor.

10. A motor speed control circuit comprising:

a rectifier, an SCR device, and a storage capacitor coupled together ina series loop circuit;

an alternating current source coupled to said rectifier;

means for coupling the motor armature in parallel with said storagecapacitor;

means coupled .in parallel with said .storage capacitor for developingfeedback signals from the motor armature; and a firing control circuitfor controlling the firing angle of said SCR device during eachhalf-cycle of voltage applied through said rectifier, said firingcontrol circuit being connected between anode and cathode of said SCRdevice for receiving operating energy when said SCR deviceisnonconductive, having an input not directly connected to said SCRdevice but coupled to said feedback developing means for receiving saidfeedback signals therefrom, and having an output connected to the gatelead of said SCR device for selectively initiating conduction of saiddevice.

11. A motor speed control circuit as claimed in claim 10 wherein saidfeedback developing means includes a resistor coupled in series with themotor armature, the series combination of said resistor and motorarmature being coupled in parallel with said storage capacitor; andmeans for deriving across said resistor a voltage signal proportional toarmature current.

12. A motor speed control circuit as claimed in claim 11 which includesa voltage divider coupled in parallel with said storage capacitor, andmeans for deriving from said voltage divider a voltage signalproportional to armature voltage.

13. A motor speed control circuit as claimed in claim 12 wherein saidfiring control circuit includes a transistor having two input electrodesto which said two voltage signals are respectively coupled, and anoutput electrode to which is coupled a load circuit including acapacitor and resistor connected in parallel.

14. A motor speed control circuit as claimed in claim 10 which includesa voltage divider coupled in parallel with said storage capacitor, andmeans for deriving from said voltage divider a voltage signalproportional to armature voltage.

7 15. A motor speed control circuit having provision for limiting thearmature current to a predetermined safe value, comprising:

a rectifier, an SCR device, and a motor armature coupled together in aseries loop circuit;

an alternating current source coupled to said rectifier;

a firing circuit for controlling the firing of said SCR device duringeach half cycle of the alternating voltage that passes through saidrectifier;

means associated with 'said armature for developing regenerativefeedback control signals;

a control circuit responsive to said control signals when the armatureload increases to actuate said firing circuit so as to increase thearmature voltage, said control circuit in-' cluding a first transistorhaving a bias current path, a second transistor having itsemitter-collector current path connected as a series portion of saidbias current path, means normally biasing said second transistor to aconductive state so that said first transistor is normally conductiveand circuit means for normally delivering actuating current through theemitter-collector path of said first to the base of said secondtransistor, a resistor connected in series with the armature in saidseries loop circuit so as to produce a control voltage proportional toarmature current, said voltage divider having one of its ends connectedto one end of said resistor, the base-emitter path of said secondtransistor being connected to the other end of said resistor.

1. A motor speed control circuit comprising: an SCR device coupled in aseries loop circuit with a motor armature for controlling the energyflow thereto; means for applying a rectified but pulsating voltageacross said series loop circuit; means for producing a first voltagefeedback signal substantially proportional to the voltage applied to themotor armature; means for producing a second voltage feedback signalsubstantially proportional to the current drawn by the motor armature;signal responsive means responsive to both of said feedback signals forproducing a composite control signal as a function of a plurality ofcycles of both of said feedback signals; and control means coupled inparallel with said SCR device and energized by the voltage existingbetween anode and cathode thereof, said control means having an inputcoupled to said signal responsive means for receiving said compositecontrol signal and having an output coupled to the gate terminal of saidSCR device for selectively initiating current flow through said SCRdevice, whereby each time that said SCR device commences to conduct thevoltage between its anode and cathode drops to about a fraction of avolt and the energizing voltage for said control means dropsaccordingly.
 2. A motor speed control circuit as claimed in claim 1wherein said second voltage feedback signal producing means includes aresistor coupled in series with the motor armature, and which furtherincludes a capacitor coupled in parallel with the series combination ofsaid armature and resistor.
 3. A motor speed control circuit as claimedin claim 2 which further includes a diode coupled in parallel with saidcapacitor and poled oppositely to said SCR device.
 4. A motor speedcontrol circuit as claimed in claim 2 wherein said signal responsivemeans includes an electron discharge device having first and secondcontrol electrodes coupled to said first and second voltage feedbacksignal producing means, respectively, and having an output electrode,and further including a storage capacitor coupled to said outputelectrode.
 5. A motor speed control circuit as claimed in claim 2wherein said first voltage feedback signal producing means includes anadditional pair of resistors coupled in series with each other and inparallel across said series combination of armature and resistor.
 6. Amotor speed control circuit as claimed in claim 4 wherein said signalresponsive means includes an electron discharge device having first andsecond control electrodes coupled to said first and second voltagefeedback signal producing means, respectively, and having an outputelectrode, and further including a storage capacitor coupled to saidoutput electrode.
 7. A motor speed control circuit as claimed in claim 1wherein said signal responsive means includes an electron dischargedevice having first and second control electrode coupled to said firstand second voltage feedback signal producing means, respectively, anDhaving an output electrode, and further including a storage capacitorcoupled to said output electrode.
 8. A motor speed control circuit asclaimed in claim 7 wherein said control means includes a unijunctiontransistor, a firing capacitor, a transistor regulating current flow tosaid firing capacitor, and a Zener diode regulating the voltage suppliedto the series combination of said firing capacitor and transistor.
 9. Amotor speed control circuit as claimed in claim 1 wherein said controlmeans includes a unijunction transistor, a firing capacitor, atransistor regulating current flow to said firing capacitor, and a Zenerdiode regulating the voltage supplied to the series combination of saidfiring capacitor and transistor.
 10. A motor speed control circuitcomprising: a rectifier, an SCR device, and a storage capacitor coupledtogether in a series loop circuit; an alternating current source coupledto said rectifier; means for coupling the motor armature in parallelwith said storage capacitor; means coupled in parallel with said storagecapacitor for developing feedback signals from the motor armature; and afiring control circuit for controlling the firing angle of said SCRdevice during each half-cycle of voltage applied through said rectifier,said firing control circuit being connected between anode and cathode ofsaid SCR device for receiving operating energy when said SCR device isnonconductive, having an input not directly connected to said SCR devicebut coupled to said feedback developing means for receiving saidfeedback signals therefrom, and having an output connected to the gatelead of said SCR device for selectively initiating conduction of saiddevice.
 11. A motor speed control circuit as claimed in claim 10 whereinsaid feedback developing means includes a resistor coupled in serieswith the motor armature, the series combination of said resistor andmotor armature being coupled in parallel with said storage capacitor;and means for deriving across said resistor a voltage signalproportional to armature current.
 12. A motor speed control circuit asclaimed in claim 11 which includes a voltage divider coupled in parallelwith said storage capacitor, and means for deriving from said voltagedivider a voltage signal proportional to armature voltage.
 13. A motorspeed control circuit as claimed in claim 12 wherein said firing controlcircuit includes a transistor having two input electrodes to which saidtwo voltage signals are respectively coupled, and an output electrode towhich is coupled a load circuit including a capacitor and resistorconnected in parallel.
 14. A motor speed control circuit as claimed inclaim 10 which includes a voltage divider coupled in parallel with saidstorage capacitor, and means for deriving from said voltage divider avoltage signal proportional to armature voltage.
 15. A motor speedcontrol circuit having provision for limiting the armature current to apredetermined safe value, comprising: a rectifier, an SCR device, and amotor armature coupled together in a series loop circuit; an alternatingcurrent source coupled to said rectifier; a firing circuit forcontrolling the firing of said SCR device during each half cycle of thealternating voltage that passes through said rectifier; means associatedwith said armature for developing regenerative feedback control signals;a control circuit responsive to said control signals when the armatureload increases to actuate said firing circuit so as to increase thearmature voltage, said control circuit including a first transistorhaving a bias current path, a second transistor having itsemitter-collector current path connected as a series portion of saidbias current path, means normally biasing said second transistor to aconductive state so that said first transistor is normally conductiveand circuit means for normally delivering actuating current through theemitter-colleCtor path of said first transistor to said firing circuit;and circuit means responsive to the armature current, as saidpredetermined safe value is approached, for interrupting said normalbiasing means to thereby open the bias current path of said firsttransistor.
 16. A motor speed control circuit as claimed in claim -5which includes a voltage divider having a midpoint connected to the baseof said second transistor, a resistor connected in series with thearmature in said series loop circuit so as to produce a control voltageproportional to armature current, said voltage divider having one of itsends connected to one end of said resistor, the base-emitter path ofsaid second transistor being connected to the other end of saidresistor.